This invention relates to a method and apparatus for log saw blade sharpening and, more particularly, to a method and apparatus which if not substantially eliminates, substantially maximizes the defect of "scalloping" characteristic of the prior art sharpeners.
The log saws to which this invention applies are widely used in conjunction with the rewinding of paper webs to develop toilet tissue and toweling. Rewinding is a well-known and long used procedure--antedating this century. A jumbo roll of paper from the paper-making machine is unwound, usually transversely perforated, and then rewound into a product having the retail size roll diameter. Until the 1950's, the web was slit longitudinally in the rewinder so as to develop the individual rolls of product. For example, in the United States, toilet tissue rolls are normally 41/2" wide, i.e., in the axial length. Thus, the slitters on a rewinder developing toilet tissue would slit the unwound web into 41/2" wide ribbons. These then were wound or "rewound" on paper cores cut to the same length and the product only needed to be packaged for being in the form for offering to the consumer.
Several significant defects attended this early process. For example, the narrow ribbons would often overwind one another or interweave causing the separate rolls to be firmly lockd together after winding. In other cases, the individual narrow webs were more likely to break, thus causing more frequent stoppages and low winder efficiency.
In the 1950's, the first log saws became available so that it was no longer necessary to slit the web on the rewinder but the entire web could be wound into a log containing a plurality of ultimate consumer rolls. For example, it is not uncommon for a web to be in excess of 100" wide which then results in the provision of more than 20 retail size rolls of toilet tissue. The log saws operated to transversely sever the wound log into retail size lengths, viz., normally 41/2" of axial length for toilet tissue and 11" axial length for kitchen toweling.
Over the years of use of log was which uniformly made use of rotating disc blades, there has been a continuing problem of sharpening. It will be appreciated that the saw disc has to remain sharp to make a clean, square cut. If this does not occur, the product is inferior and can be rejected either by the manufacturer or the customer. Thus, there have been many approaches to maintaining the blade disc in sharpened condition.
These efforts have been frustrated because of the phenomenon of scalloping.
Scalloping refers to the condition of the blade edge whereby the grinding is uneven causing an out-of-round condition. Usually the deviation from round consists of many (any where from 6 to 40) evenly spaced depressions in the blade circumference. Once the scalloping pattern is established, it will typically compound in severity. As a consequence, the disc blade very quickly is placed out of service due to poor cutting quality an violent grinding action.
One approach attempting to relieve the scalloping problem is set forth in co-owned U.S. Pat. No. 4,347,771. This has not proven to be as effective as originally contemplated and therefore the scalloping problem persisted.
Many causes of blade scalloping have been proposed. It is suggested that this is due to vibration. The saw blade can be considered as a rotating disc which, depending upon its geometry (diameter, thickness, clamp collar diameter, taper, etc.) and material, has multiple modes or frequencies of vibration. At these frequencies, the blade will easily sustain vibration if excited by an external force which has some component of the same frequency in its spectrum.
The dynamics of rotating blades have been studied both theoretically and empirically (see Lamb and Southwall "Vibrations of a Spinning Disk", Proceedings of the Royal Society 1921). For example, a 24" diameter steel saw blade, 0.095" thick with a 6" diameter collar can be calculated to have a multiplicity of modes of vibration, according to Schajer "Simple Formulas for Natural Frequencies and Critical Speeds of Circular Saws", Forest Product Journal, Volume 36, No. 2, 1986. This calculation leads to the determination of the 24" diameter steel saw blade having zero Nodal Diameters at 37 herz, 1 Nodal Diameter at 35 herz, 2 Nodal Diameters at 45 herz, 3 Nodal Diameters at 83 herz, 4 Nodal Diameters at 143 herz, 5 Nodal Diameters at 218 herz and 6 Nodal Diameters at 308 herz.
During sharpening, the blade and grindstone dynamics are excited by small disturbances such as initially out-of-round blade or initial stone-blade contact to product a spectrum of grinding forces. Because the amount of blade material removed is related to grind force, a slightly irregular surface is formed by these variable forces. Then, due to the rotation of the blade, the surface is continuously recycled past the grinder so that the frequencies corresponding to the saw/grinder natural frequencies are selectively accentuated. The surface frequency which best agrees with one of the blade-stone frequencies will be the first to develop a scalloped pattern.
For example, a blade rotating at 770 rpm with 17 scallops on the circumference produces a surface frequency of 218 hertz which coincides with the fifth mode of vibration.